Optimization Of Steel Metallurgy To Improve Broach Tool Life

ABSTRACT

A steel for use as a broachable workpiece having a substantially pearlite-free microstructure whereby the service life of the broach tool is maximized. Formation of pearlite is suppressed by alloy modification; on-line processing; off-line processing; or combinations of these techniques which control the microstructure and hardness levels. The workpiece is broached into the form of a powertrain component such as, for example, a gear or race which can be surface hardened after broaching by one of carburizing, nitriding, or induction hardening depending upon the carbon content of the steel. Disclosed are steel compositions and processing methods for making the steel, the broachable workpiece and the broached and surface hardened powertrain article.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/606,816 filed Sep. 2, 2004, entitled “Optimization of SteelMetallurgy to Improve Broach Tool Life”, which is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

A portion of the work pertaining to this subject matter was partiallyfunded by U.S. Department of Energy, under Contract No.DE-FC36-991D13819.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to ferrous metallurgy and, moreparticularly, to steel compositions and microstructures used for makingarticles such as, for example, powertrain gears, races and like partsthat are formed by broaching and hardened by carburizing or inductionhardening. Still more particularly, the invention is directed totechniques for obtaining an optimized steel metallurgy microstructurewhich provides a steel workpiece material that significantly improvesbroach tool life, thus lowering the manufacturing costs per part. Thepresent invention also relates to methods for obtaining the desiredmicrostructures and properties in the steel workpiece as well as to thefinished article.

2. Description of Related Art

Broaching is a machining technique commonly used to cut gear teeth orcam profiles for the high volume manufacture of powertrain parts, suchas for automotive transmissions and the like. The part profiles can beformed in a single broaching operation with minimal overall time, makingit ideal for such a cost-sensitive application. However, in order toaccomplish the broaching operation in a single station, the broachmachine must perform the entire roughing, shaping and finishing of thedesired part profile using one long, high-speed steel broach tool whichremoves metal from a workpiece in a single motion. The broach tool isrelatively expensive to manufacture and can only be redressed orsharpened a fixed number of times before the tool is no longer usable.The expense of the original broach tool and the redressing costs are asignificant portion of the overall manufacturing cost of the finishedpart. The precise broaching and tooling cost per manufactured part ishighly dependent upon the number of parts that can be manufacturedbetween broach tool redressings. With tooling costs over the life of ahelical broach bar on the order of $50,000 to $80,000, and total partsmanufactured on a single broach bar currently in the range of 10,000 to80,000 parts, the cost per part is typically in the range of $0.60 to$5.00. Hence, the broach tooling cost represents around 15% to 50% ofthe total manufacturing cost for a finished part. Therefore, whereasbroaching represents a time and plant space efficient method to cutprofiles into annular steel parts, the tooling cost to perform thisoperation represents a significant portion of the total manufacturingcost.

Previous attempts to improve the broach tool life by workpiece materialmodifications have included alternate material selection, varyingtraditional heat treat routes or the addition of alloy constituents toenhance machinability, such as, for example, sulfur and calcium.Producing optimized broaching performance has been inhibited in thatprior material and heat treatment selection attempts have been madewithout full knowledge of the factors affecting broach tool life. Inaddition, only limited amounts of specific inclusion additions arepermitted within these conventionally used workpiece steel grades. Theseprior attempts have provided only incremental improvements in tool life,ranging from 25% to 100% improvement, but they fall well short of thelevels of improvement provided by the present invention due to the factthat they have not produced the optimal broaching microstructure andhardness combination. The prior approaches also include lowering of thealloy content to lower the hardness level and abrasiveness of theworkpiece steel; increasing tempering temperature to lower the hardnesslevel; and lowering carbon level or changing heat treat method to allowfor the use of lower carbon, less abrasive steels. These approaches allproduce only the incremental improvements noted above.

Workpiece steels used to manufacture profiled parts for powertrain gearsand races can be broadly categorized as either carburizing/nitriding orinduction hardening types, depending upon the method used to harden theload bearing surfaces of the finished part. Steels have historicallybeen chosen broadly based upon the hardening method and subsequentcarbon level, and heat treat hardenability requirements for the hardenedsurface in an effort to minimize cost to manufacture the part. Theconventional workpiece steels so selected have been processed and/orheat treated to develop a ferrite/pearlite type microstructure,typically to a narrower aim/range within the overall broachable hardnessrange of 150 to 300 BHN. The resultant workpiece steel selection,processing and heat treatment to achieve the above microstructure andhardness has heretofore resulted in lower than optimal broach tool life,but has been considered acceptable based on historical data. Thehistorical data has not often been questioned or improved upon in asystematic manner due to the exorbitantly high costs of testingpotential new grades of workpiece steel on production equipment.

In powertrain components, such as gears, it is desirable to surfaceharden the broached gear teeth for wear resistance while maintaininglower hardness levels in the core of the gear for toughness. As alludedto above, the main techniques for obtaining localized surface hardeningare carburizing, nitriding and induction hardening, depending upon thetype of steel used to make the part. Generally, steel grades having lessthan about 0.32 wt. % C possess insufficient carbon at the surface toprovide thermal hardening. Accordingly, this steel type is usuallycarburized (or nitrided) to diffuse a carbon-enriched layer (ornitrogen) in the load bearing surface of the part to permit subsequentthermal/heat treat hardening. On the other hand, steels having greaterthan 0.32 wt. % C, such as grades having about 0.35 to 0.80 wt. % C, maybe surface hardened using the induction heating technique. The highfrequency magnetic field of the inductor heats the surface layer of thebroached part in a matter of mere seconds to a desired austenitizingtemperature of, say, 1700-1800° F. and the heated surface is thenimmediately quenched in water or other quench media. Since carburizing(and nitriding) usually is conducted in a controlled atmosphere furnacefor 5-10 hours, carburizing and nitriding are considerably moretime-consuming and expensive than the induction hardening technique.Heretofore, the higher carbon type of workpiece steel which is suitablefor induction hardening yielded a shorter broach tool life than thelower carbon carburizing type, following the trend that broach tool lifedecreases with increasing carbon content of the workpiece steel. Thus,the economic benefit enjoyed by fast induction hardening has heretoforebeen offset by the increased broach tool cost per part.

Applicants are aware of prior work conducted in the industry to providea broachable steel within the desirable broaching hardness range ofabout 150 to 240 BHN. This steel possesses a non-pearliticmicrostructure obtained by off-line thermal processing performed as ameans of optimizing the surface hardening response and not as a means ofoptimizing broach tool life. This steel, however, has a carbon range ofbetween 0.25 to 0.31 wt. % and is subjected to nitriding heat treatmentafter broaching to achieve surface hardening for transmission gears.

Applicants are further aware of prior work involving broached powertrainparts that require a high core hardness prior to broaching to developthe desired mechanical properties for the part. The hardness range forthese parts is typically from 250 to 300 BHN or higher. Most steelscannot achieve this hardness range while maintaining a pearliticmicrostructure. As a result, the most common method to increase hardnessto a level within this range is to perform an off-line heat treatment(quench and temper) to develop a non-pearlitic bainite/martensitetempered structure. The reason for obtaining this non-pearliticstructure is to develop the core hardness range and not to optimizebroaching. Indeed, at these high hardness levels of 250 to 300 BHN orhigher, broach tool life is sacrificed for high hardness.

SUMMARY OF THE INVENTION

The present invention solves the problems of the prior art by providingoptimized steel microstructures in a workpiece for the manufacture ofarticles by broaching, which yield greatly improved broach tool life.The articles are useful as automotive, truck, tractor and likepowertrain parts such as, for example, transmission gears and races.Still further, the present invention provides a steel for a workpiecethat includes carburizing, nitriding and induction hardening types whichsignificantly increases broach tool life for these steel types.

Briefly stated, the present invention contemplates an optimized steelmetallurgy for the workpiece to be broached, wherein the microstructuresubstantially eliminates the presence of pearlite in favor of bainite,martensite and/or ferrite. The present invention provides this optimalsteel metallurgy in a workpiece for broaching by at least one or more ofthe following processing techniques: (a) modification of the steel alloycomposition to suppress pearlite formation; (b) on-line thermalprocessing to suppress pearlite formation; (c) off-line heat treatmentto suppress the formation of the pearlite phase; and/or (d) combinationsof one or more of the above techniques or use of other techniques toachieve the desired microstructure comprising bainite and/or temperedmartensite with substantially no pearlite phase. A further aspect of theinvention provides a steel workpiece suitable for surface hardeningsubsequent to broaching by one of carburizing, nitriding or inductionhardening wherein the steel microstructure is substantially free ofpearlite by modification of the steel alloy composition coupled with oneor both of on-line or off-line thermal processing. A preferred hardnessrange for the workpiece material is between about 160 to 250 BHN and,more preferably, between about 170 to 245 BHN and, still morepreferably, between about 180 to 240 BHN. The workpiece steel has acarbon content of between about 0.15-0.80 wt. % and, more preferably,between about 0.18-0.70 wt. % C, which, thus, includes carburizing,nitriding and induction hardening types of steel. One aspect of theinvention contemplates a carbon content of about 0.15-0.35 wt. % andpreferably between about 0.20-0.32 wt. % C to permit the broachedworkpiece to be carburized. A still further aspect of the inventionincludes a workpiece steel for broaching having about 0.32-0.80 wt. % C,more preferably between about 0.33-0.70 wt. % C, and still morepreferably between about 0.35-0.65 wt. % C to permit the broachedworkpiece to be induction hardened. The invention includes the steelmaterial for making the workpiece to be broached, the steel workpiece tobe broached, the finished article made from the steel workpiece, as wellas the process for making same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of broach tool life versus carbon content for baselineand optimized steel workpiece materials of the present invention;

FIG. 2 is a graph of broach tool life versus hardness for the steelworkpieces tested in FIG. 1;

FIG. 3 is a graph of broach tool life versus carbon content for modifiedalloys composed primarily of fine bainite and baseline alloy workpieceshaving a ferrite/pearlite microstructure;

FIG. 4 is a graph of broach tool life versus carbon content for on-linethermally treated workpieces composed of tempered bainite and baselinealloy workpieces having ferrite/pearlite microstructures;

FIG. 5 is a graph of broach life versus carbon content for off-line heattreated workpieces composed of microstructures of the present inventionand baseline alloy workpieces having ferrite/pearlite microstructures;

FIG. 6 is a photomicrograph of a conventional steel, baseline grade 5130as rolled (Sample No. 2) exhibiting a typical microstructure containingpearlite and ferrite;

FIG. 7 is a photomicrograph of a conventional steel, baseline grade 5150normalized and tempered (Sample No. 10) exhibiting a typicalmicrostructure containing pearlite and ferrite;

FIG. 8 is a photomicrograph of a steel in an as-rolled condition (SampleNo. 20) treated according to the invention, alloy optimized, exhibitingan acicular bainite microstructure;

FIG. 9 is a photomicrograph of a steel in an interrupted and temperedcondition (Sample No. 30) treated on-line according to the inventionexhibiting a microstructure containing spheroidized bainite and ferrite;and

FIG. 10 is a photomicrograph of a steel in a quenched and temperedcondition (Sample No. 33) treated on-line in accordance with theinvention, exhibiting a microstructure containing spheroidizedmartensite.

DETAILED DESCRIPTION OF THE INVENTION

A laboratory broach testing machine was devised and built to enableeconomical and efficient testing and comparison of the effects ofmetallurgical variables on broach tool life. The laboratory testutilized a reciprocating 3-tooth, high-speed steel broach tool made fromM4 tool steel. This broach tool rapidly and efficiently cut theequivalent steel volumes that a typical broach tool contained in aproduction broach bar would cut over many parts. The laboratory testmachine and procedure were designed to closely mimic a productionbroaching environment in all possible aspects, including the broach toolmaterial and its heat treatment, the tool tooth design, the cuttingdepth per tooth, the cutting speed, the cutting lubricant type andlubricant delivery system, as well as the tool wear criterion limits.The broach tool was repeatedly pulled through the inner diameter of aring-shaped, annular test steel workpiece utilizing an indexing table toproperly position the annular test workpiece for each cut, employing thespecific parameters established for each of the listed variables. Thebroach tool was periodically measured for wear and the laboratory testwas deemed to be complete when the previously established wear criterionwas met, assigning a number of cuts to broach tool failure for eachmaterial workpiece condition.

This laboratory broach test was performed on a large variety of steeltypes and conditions, with each steel/condition rated against oneanother based upon the number of cuts to the specific wear criterionlimit. The database generated from this extensive testing reported inTable I indicates that an optimal workpiece steel broaching condition ormicrostructure/hardness exists for all steels, and that most steels arecurrently not being broached in this condition. The test also revealedthe non-optimal workpiece steel condition for broaching and,surprisingly, that most steels are currently being broached in thatnon-optimal condition or in a slightly modified version of thatcondition. In addition, the steel alloy compositions reported in TableII, steel processing and heat treatment schemes reported in Table IIhave also been developed in accordance with the present invention thatallow for development of optimized workpiece steel conditions forbroaching over a wide range of overall parameters. TABLE I HARDNESS ANDBROACH TOOL LIFE DETAILS OF THE WORKPIECE STEELS BROACH TESTED SampleMaterial Percent Material Material Brinell Broach Tool Life No. GradeCarbon Condition Microstructure Hardness (Cuts to Limit) Baseline Data -Prior Art Comparative 1 5120 0.20 As Rolled Ferrite/Pearlite 210 6200 25130 0.30 As Rolled Pearlite/Ferrite 210 1300 3 5130 0.30 NormalizedFerrite/Pearlite 176 4600 4 15V38R* 0.36 As Rolled Pearlite/Ferrite 237760 5 5135 0.36 Normalized Pearlite/Ferrite 195 2400 6 5046 0.46Normalized Pearlite/Ferrite 207 600 7 5046 0.48 Normalize/TemperPearlite/Ferrite 195 1200 8 1045 0.48 Normalized Pearlite/Ferrite 1971180 9 15V48R* 0.48 Normalized Pearlite/Ferrite 220 660 10 5150 0.52Normalize/Temper Pearlite/Ferrite 201 460 11 1552 0.53 Normalize/TemperPearlite/Ferrite 213 780 12 1552 0.53 Normalize/Temper Pearlite/Ferrite201 1400 13 10V55* 0.54 Normalized Pearlite/Ferrite 238 600 14 5060 0.60Normalized Pearlite/Ferrite 235 200 15 5160 0.60 Normalized Pearlite 25280 Optimized Data - Invention Alloy Optimized 20 4030 0.30 As RolledBainite/Ferrite 223 8200 21 4040 0.41 Rolled/Tempered Bainite/Ferrite188 9500 22 4050 0.51 Rolled/Tempered Bainite/Ferrite 208 3900 23 40600.62 Rolled/Tempered Bainite/Ferrite 210 3900 On-Line Optimized 30 51300.29 On-line/Temper Bainite/Ferrite 210 9600 31 5130 0.29 On-line/TemperBainite/Ferrite 190 8800 32 5046 0.47 On-line/Temper Bainite/Ferrite 2177600 33 5150 0.52 On-line/Temper Bainite/Ferrite 215 5200 Off-lineOptimized 40 8620 0.20 Normalized Bainite/Ferrite 177 12000 41 4027 0.27Normalized Bainite/Ferrite 176 11000 42 5046 0.47 Quench & Temper TemperMart. 207 8700 43 4150 0.50 Normalize/Temper Bainite/Ferrite 215 5800 441552 0.53 Quench/Temper Temper Mart. 195 6200*V represents vanadium modified, and R represents resulfurized

TABLE II COMPOSITIONS OF THE WORKPIECE STEELS BROACH TESTED SampleMaterial Grade No. SAE Designation C Mn S Si Cr Ni Mo Al V BaselineData - Prior Art Comparative 1 5120 0.21 0.88 0.032 0.29 0.86 0.1 0.030.033 0.002 2 5130 0.31 0.92 0.03 0.22 0.8 0.08 0.02 0.023 0.004 3 51300.3 0.92 0.029 0.22 0.81 0.1 0.03 0.022 0.003 4 15V38R* 0.36 1.37 0.0540.54 0.13 0.06 0.02 0.024 0.09 5 5135 0.31 0.96 0.028 0.23 0.78 0.110.03 0.022 0.002 6 5046 0.48 1.06 0.029 0.27 0.2 0.09 0.03 0.028 0.001 75046 0.48 1.06 0.029 0.27 0.2 0.09 0.03 0.028 0.001 8 1045 0.5 0.82 0.040.19 0.08 0.08 0.02 0.028 0.001 9 1548R* 0.48 1.4 0.062 0.25 0.12 0.080.02 0.002 0.002 10 5150 0.52 0.92 0.034 0.26 0.86 0.12 0.08 0.028 0.00211 1552 0.53 1.43 0.028 0.27 0.14 0.13 0.035 0.023 0.002 12 1552 0.531.43 0.028 0.27 0.14 0.13 0.035 0.023 0.002 13 10V55* 0.53 0.9 0.0320.25 0.16 0.08 0.02 0.032 0.12 14 5060 0.61 0.85 0.025 0.25 0.5 0.090.03 0.033 0.001 15 5160 0.61 0.81 0.012 0.26 0.8 0.1 0.03 0.03 0.002Optimized Data - Invention Alloy Optimized** 20 4030 0.3 0.86 0.019 0.250.11 0.11 0.25 0.025 0.001 21 4040 0.41 0.91 0.022 0.25 0.11 0.11 0.250.026 0.001 22 4050 0.51 0.89 0.021 0.25 0.11 0.11 0.25 0.03 0.001 234060 0.62 0.89 0.022 0.25 0.11 0.11 0.26 0.029 0.001 On-line Optimized30 5130 0.3 0.86 0.033 0.18 0.84 0.11 0.04 0.032 0.002 31 5130 0.3 0.860.033 0.18 0.84 0.11 0.04 0.032 0.002 32 5046 0.47 1.05 0.032 0.26 0.190.1 0.03 0.034 0.001 33 5150 0.52 0.92 0.034 0.26 0.86 0.12 0.08 0.0280.002 Off-line Optimized 40 8620 0.21 0.87 0.018 0.28 0.57 0.64 0.210.03 0.001 41 4027 0.27 0.83 0.03 0.23 0.18 0.07 0.27 0.026 0.001 425046 0.47 1.05 0.032 0.26 0.19 0.1 0.03 0.034 0.001 43 4150 0.5 0.880.024 0.27 0.98 0.11 0.18 0.033 0.002 44 1552 0.51 1.5 0.031 0.23 0.140.12 0.03 0.029 0.001*V represents vanadium modified, and R represents resulfurized.**Alloy optimized grade designations created by the inventors.

TABLE III PROCESSING DETAILS OF THE WORKPIECE STEELS BROACH TESTEDSample Material Grade No. SAE Designation Material Condition SteelProcessing* Baseline Data - Prior Art Comparative 1 5120 As Rolled Hotrolled, air cooled tubing 2 5130 As Rolled Hot rolled, air cooled tubing3 5130 Normalized Normalized tubing 4 15V38R As Rolled Hot rolled, aircooled bar, bored ID 5 5135 Normalized Normalized tubing 6 5046Normalized Normalized tubing 7 5046 Normalize/Temper Normalized tubing,tempered at 1175° F., 2 hours 8 1045 Normalized Normalized tubing 91548R Normalized Normalized tubing 10 5150 Normalize/Temper Normalizedtubing, tempered at 1330° F., 2 hours 11 1552 Normalize/TemperNormalized tubing, tempered at 1275° F., 2 hours 12 1552Normalize/Temper Normalized tubing, tempered at 1275° F., 2 hours 1310V55 Normalized Normalized tubing 14 5060 Normalized Normalized tubing15 5160 Normalized Normalized tubing Optimized Data - Invention AlloyOptimized 20 4030 As Rolled Hot rolled, air cooled 21 4040Rolled/Tempered Hot rolled, air cooled, tempered at 1300° F., 1 hour 224050 Rolled/Tempered Hot rolled, air cooled, tempered at 1300° F., 1hour 23 4060 Rolled/Tempered Hot rolled, air cooled, tempered at 1340°F., 4 hrs On-line Optimized 30 5130 On-line/Temper Austenitized tubing,interrupt water quench, tempered at 1225° F., 2 hours 31 5130On-line/Temper Austenitized tubing, interrupt water quench, tempered at1275° F., 2 hours 32 5046 On-line/Temper Austenitized tubing, interruptwater quench, tempered at 1325° F., 2 hours 33 5150 On-line/TemperAustenitized tubing, interrupt water quench, tempered at 1340° F., 4hours Off-line Optimized 40 8620 Normalized Normalized tubing 41 4027Normalized Normalized tubing 42 5046 Quench/Temper Austenitized Tubing,water quenched, tempered at 1325° F., 2 hours 43 4150 Normalize/TemperNormalized tubing, tempered at 1340° F., 3 hours 44 1552 Quench/TemperAustenitized tubing, water quenched, tempered at 1 SOOT, 4 hours*Austenitizing performed at standard temperatures (based on carbonlevel) and times (based on section size), and air cooling for normalizedentries. Hot rolling performed at standard temperatures, based on carbonlevel. Interrupt quench was sufficient to avoid pearlite formation, andvaried in time based on grade and section size.

The steels tested included composition ranges as follows, in % byweight: C - 0.18% to 0.62% Mn - 0.55% to 1.60% Si - 0.15% to 0.70% S -0.010% to 0.060% Cr - 0.10% to 1.0%  Ni - 0.05% to 0.55% Mo - 0.02% to0.30% V - 0.01% to 0.12% Al - 0.002% to 0.035% Fe - Balance, plus traceimpurities or additions of ≦0.05 each.

The steel conditions tested included: as hot worked and air cooled; hotworked and slow cooled; fine and coarse grain normalized; fine andcoarse grain normalized and tempered; quenched and tempered; interruptedquench and tempered; and annealed. Typical parameters for theseconditions are as follows.

The steel samples (Nos. 1-15, 30-33 and 40-44) reported in Tables I-IIIwere melted in a production electric furnace operation and ladle refinedin accordance with the compositions set forth in Table II. The steelsamples were continuously cast into blooms and then heated to 2250° F.,hot rolled into round billets, subsequently reheated to 2250° F. andpierced to provide an as-rolled tubular shape. The tube was then cut toform a ring-shaped, annular steel workpiece for testing or for furtherthermal treatment in accordance with Table III prior to test broaching.The resultant microstructures for the various steel samples, as reportedin Table I, included a mixture of ferrite, pearlite, bainite and/ormartensite in both tempered and non-tempered versions with varyingdegrees of carbide spheroidization. The hardness levels of the varioussteels tested ranged from 150 BHN to 330 BHN, with most of the hardnesslevels being in the broachable range from 160 BHN to 260 BHN. As shownin Table I, a presently preferred broachable hardness range is fromabout 150 to 250 BHN and, more preferably, between 175 to 240 BHN. Theanalysis of the wide range of steel compositions, shown in Table II,processing schemes (Table III), microstructure and hardness levels(Table I) has enabled the identification of the critical metallurgicalvariables to optimize broach tool life according to the presentinvention.

Historical broaching knowledge and expertise has heretofore indicatedthat higher carbon contents and hardness levels negatively impact broachtool life with traditional processing methods. Those results aresupported in the broach test data generated (Table I), where thebroaching results for numerous conventional “baseline” workpiece steelgrades, Sample Nos. 1 to 15, are shown as a function of carbon levelranging from 0.20 wt. % to 0.60 wt. % (FIG. 1) and hardness level (FIG.2).

More specifically, it will be seen in FIG. 1 and in Table I that abaseline workpiece steel (Sample No. 1) having a carbon content of 0.20wt. % and a Brinell hardness of 210 BHN yielded a broach tool life of6,200 (cuts to limit). As the baseline steel carbon content increased to0.6 wt. % (Sample No. 15), the Brinell hardness rose to 252 BHN and thebroach tool life decreased to 80 (cuts to limit). The microstructure ofthese conventional baseline workpiece steel grades was predominantlyferrite and pearlite, as shown in FIG. 6 and in FIG. 7. In thephotomicrographs of FIGS. 6 and 7, the ferrite appears as the lightregions and the pearlite appears as the darker regions. FIG. 6 shows abaseline 5130 grade steel (Sample No. 2) microstructure containingferrite and pearlite in the as rolled condition and FIG. 7 depicts abaseline 5150 grade (Sample No. 10) microstructure containing ferriteand pearlite in the normalized and tempered condition. These ferrite andpearlite microstructures are typically specified as an acceptablebroaching microstructure for most conventional broaching applications.

In contrast, the present invention encompasses the discovery that at thesame carbon and hardness levels as shown in the baseline steels in TableI, broach tool life can be increased by 2 to 10 times by altering somestage of the metallurgical processing to suppress or minimize formationof the pearlitic microstructure and, in its place, forming a fineracicular carbide microstructure which preferably may tend towardsspheroidization. The preferred microstructure of the invention issubstantially pearlite-free and consists of either predominantly bainiteand/or martensite and ferrite, and may be tempered into a desiredhardness range of 160 to 260 BHN at higher carbon levels. Some pearlitephase may be present, up to a maximum of about 20% lamellar pearlite byvolume, and preferably no more than 10% lamellar pearlite by volumemaximum, and still more preferably no more than 5% lamellar pearlite byvolume. Ideally, substantially no lamellar pearlite is present in themicrostructure. As used herein, the phrase “substantially pearlite-free”or “minimal pearlite” means a microstructure containing up to about 20%by volume lamellar pearlite and, more preferably, 0%, unless otherwisequalified.

A desired acicular bainite microstructure is shown in FIG. 8; a desiredspheroidized bainite microstructure is shown in FIG. 9; and a desiredspheroidized martensite and ferrite microstructure is shown in FIG. 10.FIG. 8 is an alloy modified 4030 grade (Sample No. 20) in the as rolledcondition; FIG. 9 is an on-line quenched and tempered 5130 grade (SampleNo. 30); and FIG. 10 is an on-line quenched and tempered grade 5150steel (Sample No. 33), all produced according to the present invention.

The substantially pearlite-free microstructures of the inventioncontaining one or more of bainite, martensite, and ferrite with zero orminimal pearlite are formed by a variety of techniques including, butnot necessarily limited to, (1) modification of the alloy makeup tosuppress pearlite formation in the hot worked, air-cooled condition(Sample Nos. 20-23); and/or (2) on-line hot processing to suppress thepearlite formation on cooling from hot working temperatures (Sample Nos.30-33); and/or (3) off-line heat treatment to suppress the pearliteformation (Sample Nos. 40-44); or by some combination of thesetechniques, or by other techniques such as by isothermal transformationbelow pearlite formation temperatures. The finalbainite/martensite/ferrite, substantially pearlite-free microstructurecan, of course, then be tempered into the desired broaching hardnessrange, if desired.

The resultant effect of this microstructure modification can be observedby comparing the previously mentioned broach tool life trends forvarious carbon and hardness levels having a predominantly pearliticmicrostructure with the optimized broach tool life results for similarsteels substantially pearlite-free within the same carbon levels(FIG. 1) and hardness ranges (FIG. 2). As can be readily noted, theimprovement in broach tool life is on the order of 200% to 1000%,comparing the same or similar initial carbon contents and broach toollife, all within similar hardness ranges. For example, the broach toollife of prior art baseline 5120 grade workpiece steel (Sample No. 1)having 0.20% C with a ferrite/pearlite microstructure yielded a broachtool life of 6200 (cuts to limit). This may be compared with off-linenormalized 8620 grade (Sample No. 40), also having a 0.20% C content,but with a bainite/ferrite microstructure of the invention whichprovided a broach tool life of 12,000 (cuts to limit), providing nearlydouble the broach tool life of the conventional workpiece steel. Similarimprovements in broach tool life realized with increasing carboncontents are seen graphically in FIG. 1. This significant level ofbroach tool life improvement directly translates into a large reductionin broach tooling cost per part. More specifically, the presentinvention provides a 40% to 80% reduction in tooling cost per part, and,thus, allows the part manufacturer to realize a significant overallreduction in the part manufacturing cost. As alluded to above, this costsavings is of utmost importance in the production of high volumeprecision automotive parts.

Obtaining the optimal steel workpiece condition for broaching can berealized in accordance with the invention by any individual orcombination of the three aforementioned techniques including, but notnecessarily limited to: (1) modification of the alloy makeup to suppressor minimize pearlite formation in the hot worked, air cooled condition;(2) designing the on-line hot processing to suppress the pearliteformation on cooling from the hot working temperature; and/or (3)off-line heat treatment to suppress the pearlite formation, followed bya temper operation, when necessary.

Other techniques for the suppression of the unwanted formation of apearlite microstructure may occur to persons of skill in the art andthose techniques will fall within the spirit and scope of the presentinvention. Examples of the presently preferred inventive techniques arediscussed hereinafter.

1. Alloy Modification

Alloy compositions can be designed to suppress pearlite formation by theaddition of several potential chemical elements and addition levels,individually or in combinations, depending upon the base steelcomposition. An example of the alloy modification approach wasdemonstrated by melting a series of laboratory vacuum induction meltedheats at four carbon levels (0.3% C, 0.41% C, 0.51% C and 0.62% C),where approximately 0.25% Mo was added to a base carbon and manganesecomposition to suppress the pearlite formation. It will be understood,unless noted otherwise, that all percentages are in % by weight. Morespecifically, the alloy compositions for these sample Nos. 20-23,designated grades 4030, 4040, 4050 and 4060, respectively, had thecompositions shown in Table II with carbon levels ranging from 0.30 to0.62 wt. %. As seen in Table III, the ingots were hot rolled andair-cooled, fashioned into test rings and tempered as necessary toachieve a desired broaching hardness range of between about 180-240 BHN(Table I). The microstructure of these steels was primarily composed ofa fine bainite/ferrite. The microstructure tended to spheroidization, ifthe steel had been tempered as in Sample Nos. 21-23. An example of aspheroidized bainite microstructure is shown in FIG. 9.

FIG. 3 graphically depicts the broach test results for the alloymodified inventive steels (Sample Nos. 20-23) in comparison to aconventional baseline set of ferrite/pearlite containing steels (Samples3, 5, 7, 11 and 14) covering a similar carbon range of 0.3 to 0.60% C.The baseline steels exhibit a steady decrease in broach tool life withincreasing carbon level from 4600 cuts to limit at 0.30% C (Sample No.3), down to only approximately 200 cuts to limit at 0.60% C (Sample No.14). However, the alloy modified steels of the invention show 9500 cutsto limit at 0.41% C (Sample No. 21), and then nearly 4000 cuts to limitat 0.62% C (Sample No. 23). The modified steel composition of thepresent invention shows well in excess of an order of magnitudeimprovement in broach tool life over the conventional baseline steel atcomparable carbon levels. Therefore, an alloying modification approachof the invention to optimize the microstructure and broach tool life ofgrades with varying carbon levels has been demonstrated to be successfuland effective, as evidenced by the test data.

Presently preferred modified steel compositions of the present inventionwhich suppress or minimize the formation of a microstructure containingpearlite in the hot rolled, air cooled condition (with optionaltempering) are listed below in Table IV in % by weight. TABLE IV Element(wt. %) Broad Range Mid-Range Narrow Range Nominal C  0.2-0.80 0.25-0.650.35-0.6  0.3-0.6 Mn  0.5-1.75 0.7-1.5 0.80-1.30 1.0 S 0.010-0.100.015-0.07  0.018-0.030 0.025 Si  0.10-0.70 0.15-0.35 0.20-0.30 0.25 Cr 0.01-0.50 0.03-0.35 0.05-0.25 0.1 Ni 0.01-1.0 0.03-0.25 0.05-0.20 0.1Mo  0.10-0.50 0.15-0.40 0.20-0.30 0.25 Al 0.001-0.07 0.01-0.050.015-0.04  0.03 V 0.001-0.20 0.01-0.10 0.01-0.03 0.001 B 0.005-0.030.007-0.025 0.010-0.02  0.015 Fe Balance* Balance* Balance* Balance**Balance iron plus incidental additions and impurities less than 0.05%each.

More specifically, presently preferred alloy modified compositions forachieving the desired properties of the invention are also set forth inTable II, for Sample Nos. 20-23, appearing under the heading “AlloyOptimized”.

2. On-Line Processing

The second technique for improving broach tool life according to theinvention involves on-line processing to achieve the desiredmicrostructure. On-line thermal treatment involves hot working andcooling, using thermomechanical processing schemes that can be devisedwhereby the unwanted pearlite phase is suppressed upon cooling from hotworking, such as hot rolling, without having to modify the steelcomposition at all or to a lesser extent. The term “on-line”optimization or processing as used herein means a thermomechanical steelprocess scheme directly coupled with the final hot working operation(rolling, forging, etc.) whereby the pearlite phase is avoided orminimized and the bainite/martensite/ferrite phases are promoted.

An example of this on-line technique was demonstrated by austenitizingat 1600° F. grades 5130 and 5046 steel tubing, Sample Nos. 30 and 32,respectively, to varying austenite grain structures (simulating thefinal as hot worked microstructure range), followed by performing aninterrupted quench in water to avoid pearlite formation and thentempering at 1225° F. for 2 hours (Sample No. 30) and tempering at 1325°F. for 2 hours (Sample No. 32) to bring the steel into the desiredhardness range (about 180-240 BHN). The microstructures and hardnesslevels of both grades were composed of a tempered bainite tending tospheroidize, with hardnesses in the range of 200 to 235 BHN. FIG. 4shows the results of the broach testing performed on the on-lineprocessed grades of the invention in comparison to the variousconventional baseline ferrite/pearlite grades tested and shownpreviously in FIG. 3 (Sample Nos. 3, 5, 7, 11 and 14). The broach toollife test results for the on-line optimized grades 5130 and 5046 (SampleNos. 30 and 32) were two to six times greater than the baseline levelreported, which illustrates the success of this approach to optimizebroach tool life for both the carburizing and induction hardening gradesof steels. It will be noted that grade 5130 (Sample No. 30) contained0.29 wt. % C and is a carburizing type of steel, while grade 5046(Sample No. 32) contained 0.47 wt. % C and is an induction hardeningtype of steel.

By way of direct comparison of nearly identical compositions regardingthe baseline grades 5130 (Sample Nos. 2 and 3) and grade 5046 (SampleNos. 6 and 7) with on-line optimized grades 5130 (Sample Nos. 30 and 31)and grade 5046 (Sample No. 32), attention is directed to Tables I, IIand III. The on-line optimized Sample No. 30 grade 5130 (0.29 wt. % C)steel had a broach tool life of 9600 cuts to limit and Sample No. 31 hada broach tool life of 8800 cuts to limit compared to the baseline grade5130 with a life of only 1300 cuts to limit (Sample No. 2 as rolled) and4600 cuts to limit (Sample No. 3, normalized), both containing 0.30 wt.% C. Higher carbon level conventional grade 5046 (Sample No. 6)containing 0.46 wt. % C in the normalized condition had a broach toollife of 600 cuts to limit compared with on-line optimized grade 5046(Sample No. 32) also containing 0.46 wt. % C, on-line processed andtempered, had a broach tool life of 7600 cuts to limit, which representsmore than a twelvefold increase in broach tool life.

3. Off-Line Processing

A further inventive technique for obtaining a desired microstructure ina broachable workpiece involves off-line processing. The term “off-line”processing or optimization as used herein means a thermal processingmethod performed at some time subsequent to hot working whereby thepearlite phase is substantially avoided and thebainite/martensite/ferrite phases are promoted. The off-lineoptimization scheme can be performed on a wide variety of steel typesand section sizes to totally suppress or otherwise minimize pearliteformation. Such thermal treatments are most often followed by temperinginto the appropriate broaching hardness range, which typically is 180 to240 BHN. Off-line heat treatments according to the invention usuallyinvolve the following steps: austenitizing the steel test section at1500° F. to 1750° F.; fully or interrupted quenching of the steelsection in the appropriate quench media, such as water, to avoidpearlite formation based on classical steel hardenability calculations.Quenching is normally followed by tempering at 1100° to 1350° F. for 1to 4 hours (depending on steel type and temperature) to form the desiredmicrostructure (substantially free of pearlite phase) and to provide adesired hardness for broaching between about 180-240 BHN.

Table I shows that off-line optimized Sample Nos. 40 (grade 8620)containing 0.20 wt. % C and 41 (grade 4027) containing 0.27 wt. % C werein the normalized condition and provided the highest broach tool life ofany of the samples tested, viz., 12,000 cuts to limit and 11,000 cuts tolimit, respectively. Normalizing involves heating the steel to atemperature above the transformation range and then cooling in still airat room temperature. The resultant microstructure for Sample Nos. 40 and41 was a bainite/ferrite with no pearlite in the microstructure.

Sample No. 43 (grade 4150) containing higher carbon at 0.50 wt. % C wasalso subjected to a normalizing treatment and subsequently tempered at1340° F. for 3 hours and, likewise, exhibited a bainite/ferritemicrostructure, free of pearlite. The workpiece from Sample No. 43yielded a broach tool life of 5,800 cuts to limit in the broaching test,compared to 460 cuts to limit for Sample No. 10 (conventional grade 5150steel) of comparable carbon content and hardness (Table I) to Sample No.43.

Off-line optimized Sample No. 42 (grade 5046) containing 0.47 wt. % Cand Sample No. 44 (grade 1552) containing 0.53 wt. % C wereaustenitized, water quenched, and then tempered at 1325° F. for 2 hoursfor Sample 42 and tempered at 1300° F. for 4 hours for Sample No. 44.The quenching and tempering produced a tempered martensiticmicrostructure free from pearlite according to the present invention.Broach tool life was 8,700 cuts to limit for Sample No. 42 and 6,200cuts to limit for Sample No. 44, which is a significant improvement inthese high carbon, induction hardening steel types.

FIG. 5 shows the broach tool life for each of these steels versus thebaseline ferrite/pearlite grades, which illustrates the same trends withcarbon level and similar or greater levels of improvement over thebaseline grades, as compared to the other techniques. Accordingly, itwill be understood that the off-line processing optimization approachrepresents another technique to optimize broach tool life according tothe present invention.

4. Combinations of the Optimization Techniques

Two or more of the above-discussed optimization techniques 1-3 can becombined to provide the microstructure and properties desired in abroachable workpiece according to the present invention.

For example, Sample Nos. 40 and 41 were off-line treated grades 8620 and4027, discussed above, and were also subjected to alloy optimizationwith Mo additions: 0.21 wt. % Mo (Sample No. 40) and 0.27 wt. % Mo(Sample No. 41), see Table II. These off-line normalized alloy modifiedsteels provided the highest broach tool life of all samples tested, thusevidencing the effectiveness of the combined alloy and heat treatmentoptimization techniques of the invention.

Surface Hardening

As discussed hereinabove, it is beneficial in powertrain components toprovide a hardened outer surface for improved wear resistance of thebroached gear teeth and a lowered hardness core to provide internaltoughness to the part. Known techniques for surface hardening ofbroached powertrain components include carburizing, nitriding andinduction hardening, and the particular technique employed is dependentupon the carbon content of the steel. Generally, steels containing lessthan about 0.32 wt. % C have insufficient carbon levels for thermallyinduced surface hardening as provided by induction hardening. Thus,these steels require additional carbon or other constituent to permitsurface hardening. Steels having carbon contents less than about 0.32wt. % are generally carburized or nitrided to obtain enhanced surfacehardening.

The above-discussed test results clearly demonstrate that the presentinvention provides a marked improvement in broach tool life forbroaching a variety of workpiece steels, and some aspects of alloymodifications and material process schemes by which these improvementsare realized have been described herein. The target broaching workpiecemicrostructure has been shown to be a non-pearlitic, fine carbidebainite/martensite microstructure ideally tending towardsspheroidization. Alloy and process schemes to develop the targetmicrostructure/hardness combinations in the workpiece steels have beenshown and described in connection with the above sample steels. The dataindicate that the present invention is applicable over a wide range ofsteel compositions and processing alternatives, given that the finaltarget metallurgical characteristics are achieved. It will occur tothose skilled in the art that there may be alternate alloyingmodifications and/or steel processing schemes in addition to thespecific examples set forth above that can be used to obtain the targetnon-pearlitic, fine carbide microstructure, preferably tending towardsspheroidization. It is, of course, understood that those alternateand/or additional methods would also be expected to achieve thebeneficial broach tool life results set forth herein, and that suchadditional modifications are deemed to fall within the spirit and scopeof the present invention.

1. A steel suitable for making a broachable workpiece, said steel havinga substantially pearlite-free microstructure and a hardness of between150 to 250 BHN prior to broaching and subsequent surface hardening byone of carburizing or induction hardening.
 2. The steel of claim 1wherein the microstructure comprises one or more of bainite, martensiteand ferrite phases and a hardness of between 160 to 240 BHN.
 3. Thesteel of claim 1 wherein said steel has a carbon content between0.15-0.35 wt. % and is suitable for carburizing after broaching.
 4. Thesteel of claim 1 wherein said steel has a carbon content between0.32-0.80 wt. % and is suitable for induction hardening after broaching.5. A steel workpiece for broaching, said steel workpiece having asubstantially pearlite-free microstructure and a hardness of between 150to 250 BHN, wherein said workpiece is suitable for surface hardeningsubsequent to broaching by one of carburizing or induction hardening. 6.The steel workpiece of claim 5 wherein the microstructure comprises oneor more of bainite, martensite and ferrite phases and a hardness ofbetween about 160 to 240 BHN.
 7. The steel workpiece of claim 5 whereinsaid steel is one of a carburizing type having a carbon content ofbetween 0.15-0.35 wt. %.
 8. The steel workpiece of claim 5 wherein saidsteel is an induction hardening type having a carbon content of betweenabout 0.35 to 0.80 wt. %.
 9. A broached steel article suitable for useas a powertrain component, said article having a substantiallypearlite-free microstructure and a broached surface profile wherein saidsurface profile has a hardened surface applied by one of carburizing orinduction hardening.
 10. The article of claim 9 in the form of one of agear or race.
 11. The article of claim 9 wherein the microstructurecomprises one or more of bainite, martensite and ferrite phases.
 12. Thearticle of claim 11 in the form of one of a gear or race.
 13. Thearticle of claim 9 wherein said steel has a carbon content of between0.20-0.32 wt. % and is surface hardened by carburizing.
 14. The articleof claim 13 in the form of one of a gear or race.
 15. The article ofclaim 9 wherein said steel has a carbon content of between about 0.33 to0.70 wt. % and is surface hardened by induction hardening.
 16. Thearticle of claim 15 in the form of one of a gear or race.
 17. A steelcomposition suitable for broaching comprising, in % by weight:0.15-0.65% C; 0.5-1.75% Mn; 0.010-0.10% S; 0.10-0.70% Si; 0.1-0.50% Cr;0.01-1.0% Ni; 0.10-0.50% Mo; 0.001-0.07% Al; 0.001-0.20% V; 0.005-0.03%B; and balance Fe plus incidental additions and impurities, less than0.05% each.
 18. The steel of claim 17 comprising 0.20-0.65% C; 0.7-1.5%Mn; 0.015-0.07% S; 0.15-0.35% Si; 0.03-0.35% Cr; 0.03-0.25% Ni;0.15-0.40% Mo; 0.01-0.05% Al; 0.01-0.10% V; and 0.007-0.025% B.
 19. Thesteel of claim 17 comprising 0.3-0.6% C; 0.80-1.30% Mn; 0.018-0.030% S;0.20-0.30% Si; 0.05-0.25% Cr; 0.05-0.20% Ni; 0.20-0.30% Mo; 0.015-0.04%Al; 0.01-0.03% V; and 0.010-0.02% B.
 20. The steel of claim 17comprising 0.3-0.6% C and nominally: 1.0% Mn; 0.025% S; 0.25% Si; 0.1%Cr; 0.1% Ni; 0.25% Mo; 0.03% Al; 0.001% V; and 0.015 B.
 21. The steel ofclaim 17 in one of a hot worked or normalized condition containingbainite, martensite and/or ferrite and having a substantiallypearlite-free microstructure and suitable for surface hardening by oneof carburizing or induction hardening.
 22. A method for making aworkpiece material to be subjected to broaching and subsequent surfacehardening by one of carburizing or induction hardening, comprising thesteps of: (a) providing a steel; and (b) subjecting the steel to one ormore treatments comprising: alloy modification, on-line thermaltreatment, and off-line thermal treatment to thereby provide a steelhaving a substantially pearlite-free microstructure and a hardnessbetween 150 to 250 BHN prior to broaching.
 23. The method of claim 22wherein the alloy modification treatment comprises adding one or morealloying elements to the steel for suppressing the formation of pearlitein the steel.
 24. The method of claim 23 wherein Mo is added to thesteel in the amount of 0.15 to 0.40 wt. % to suppress the formation ofpearlite.
 25. The method of claim 22 wherein the on-line thermaltreatment includes the steps of austenitizing and interrupt quenching toproduce a microstructure containing bainite martensite and/or ferrite.26. The method of claim 25 including the step of tempering after thequenching step.
 27. The method of claim 26 wherein the tempering step isconducted at a temperature of 1100° F. to 1340° F. for 1 to 4 hours. 28.The method of claim 22 wherein the off-line thermal treatment includesone of normalizing, normalizing and tempering, and austenitizing,quenching and tempering.
 29. The method of claim 28 wherein the steel isone of normalized or normalized and tempered to produce a microstructurecontaining bainite and ferrite.
 30. The method of claim 28 wherein thesteel is austenitized, quenched and tempered to produce a microstructurecontaining tempered martensite.
 31. The method of claim 28 wherein thetempering in the off-line thermal treatment is conducted at 1200° to1340° F. for 1 to 4 hours.
 32. The method of claim 22 including the stepof forming the steel into a hot rolled tubular shape to provide aworkpiece for broaching powertrain gears and races.
 33. The method ofclaim 22 wherein said steel is subjected to at least two of saidtreatments.
 34. A method for making a powertrain component comprisingthe steps of: (a) providing a molten steel having a carbon contentbetween about 0.15 to 0.80 wt. % C and optionally subjecting the moltensteel to an alloy modification treatment to suppress the formation of apearlite phase; (b) subjecting the steel to one or more treatmentsincluding on-line thermal treatment and off-line thermal treatment tosuppress the formation of the pearlite phase; (c) providing a steelworkpiece for broaching having a substantially pearlite-freemicrostructure and a hardness between 150 to 250 BHN; (d) broaching thesteel workpiece to form a broached powertrain component; and (e) surfacehardening the broached powertrain component by selecting one ofcarburizing or induction hardening.
 35. The method of claim 34 whereinthe steel contains between 0.2 to about 0.32 wt. % C and the surfacehardening step selected is carburizing.
 36. The method of claim 35wherein the steel contains greater than 0.32 wt. % C up to 0.65 wt. % Cand the surface hardening step selected is induction hardening.
 37. Themethod of claim 34 wherein the optional alloy modification treatment instep (a) comprises adding 0.10 to 0.50 wt. % Mo to the molten steel. 38.A method for making a powertrain component comprising the steps of: (a)providing a molten steel having a carbon content between about 0.15 to0.80 wt. % C; (b) subjecting the molten steel to an alloy modificationtreatment to suppress the formation of a pearlite phase; (c) subjectingthe steel to one or more of on-line thermal treatment and off-linethermal treatment to suppress the formation of a pearlite phase andprovide a workpiece for broaching having a substantially pearlite-freemicrostructure; (d) broaching the workpiece to form a broachedpowertrain component; and (e) surface hardening the broached powertraincomponent by one of carburizing, nitriding or induction hardening. 39.The method of claim 38 wherein the steel contains between 0.2 to about0.32 wt. % C and the surface hardening step is one of carburizing ornitriding.
 40. The method of claim 38 wherein the steel contains between0.35 to 0.65 wt. % C and the surface hardening step selected isinduction hardening.